Fusible impregnation of porous metallic bodies



2,971,865 Patented Feb. 14, 1961 FUSIBLE IMPREGNATION OF POROUS METALLIC BODIES Arthur G. Metcalfe, Park Forest, and Harvey B. Nudelman, Chicago, 111., assignors to the United States of America as represented by theSecretary of the Army N Drawing. Filed Mar. 15, 1957, Ser. No. 646,503

1 Claim. (Cl. 117-130) The present invention pertains to a method of decreasing the porosity of metallic bodies and particularly to a method of impregnating and sealing metallic bodies with a fusible filler.

Castings, especially sand castings, contain minute voids or pores within thecasting metal itself which renders the casing porous to a limited degree. Although the porosity of a metal casting or body is usually small and does not become objectionable for most uses of the casting, there are particular applications wherein any flow of gas or fiuid through the casting body is highly undesirable. castings are utilized in high or very low pressure applications, especially high pressure usages such as encountered in some fluid and gas circuits. Therefore valves, conduits, pumps, gages, and other components of high pressure fluid or gas circuits must be of such homogeneous structure as to prevent escape of the pressure through the metal itself. Such requirement for low porosity components of high pressure systems has necessitated the use of dense and expensive metals.

To decrease the cost of components used with high pressure systems, a procms of treating low cost castings and other porous metallic bodies has been developed wherein the body is impregnated with a material which will close the minute openings within the metal body thereby decreasing the porosity. After impregnation, the casting is usually exposed to some type of heat treatment to increase the eifectiveness of the sealing action. It is to this type of method for decreasing metallic porosity to which the invention is directed.

It is thus an object of the invention to produce a method of impregnating metal with a filler which will fuse with the base metal decreasing the porosity thereof.

Another object of the invention is to produce an efiicient system of impregnating a metal body with a stoichiometric Thermit reaction mixture whereby upon heat treatment the mixture will fuse With the impregnated metal.

Another object of the invention is to impregnate a metal body with such Thermit reaction mixture and heat said body whereby the Thermit reaction will take place closing the pores of the body.

These and other objects of the invention will become apparent when considered with regard to the following description and appended claim.

A simplified flow diagram of the process hereinafter described in detail is as follows:

Porosity in metals becomes a problem when Porous metal body Impregnator, suspension of reaction mixture in volatile liquid Subatmospheric pressure Superatmospheric pressure impregnation Heat treatment Volatile liquid vapor Sealed metal product The base metal utilized by the invention will most likely consist of a low cost iron alloy or steel casting which contains minute voids o'r pores between the molecular structures which permit fluid or gas to slowly leak through the casting. To close the pores, it is contemplated by the invention to fill the pores or voids with a stoichiometric reaction mixture of finely powdered aluminum and ferric oxide, cause the mixture to react forming free iron which will fuse to the base metal and close the pores against passage of high pressure fluids or gases.

A suitable reaction mixture may consist of a suspension consisting of a mixture of finely powdered 30.2% Fe O and 10.0% al suspended in 59.8% industrial ethyl alcohol, all by weight. The ferric oxide and aluminum must be finely powdered to pass into the pores of the metal and may range between 325-500 mesh size. The ethyl alcohol does not form a part of the reaction and serves to provide a non-corrosive, low-boiling point ve hicle for the reacting elements which is easily removed from the casting after impregnation. Thus, it will be understood that the resulting Thermit mixture produces the reaction Fe O +2al Al O -{-2Fe whereby the resultant aluminum oxide and iron fuse to the base metal providing the sealing action.

Preferably the impregnation of the casting or other metallic body with the Thermit composition is accomplished through a combination vacuum and pressure treatment to insure efiective infiltration. The casting to be treated is placed within an air-tight chamber which is connected to a vacuum pump. The chamber is evacuated to subatmospheric pressures and while the vacuum is maintained the suspension mixture of ferric oxide and aluminum powder suspended in the ethyl alcohol is drawn into the chamber until the body is completely immersed in the suspension. While the body is completely covered with the impregnating mixture, the subatmospheric pressure is released and the chamber is subjected to compressed air of approximately p.s.i. for five minutes or more. The body may now be removed from the chamber.

During the treatment in the chamber, the casting or metal body will absorb the Thermit mixture as the combination vacuum and high pressure exposure both draw and push the mixture into the pores and voids of the body, thus a deep and efiective impregnation is achieved.

The metal body must be exposed to heat treatment to initiate the desired reaction of the Thermit mixture and therefore after removal from the chamber the body is fired at elevated temperatures. Best results are obtained at a firing temperature of 1100 F. for one hour although complete reaction will occur at 1000 F. if fired for a longer period of time.

It is believed that due to the small bulk of the Thermit mixture the reaction may take place slower than in the Thermit process used in welding metal. Also the relative high surface to volume ratiomay tend to remove heat from the reaction mixture preventing the spontaneous reaction which occurs in the bulk state, thus a relatively long heat treatment is required. 7

During the early stages of the firing process the ethyl alcohol will be evaporated leaving only the dry powders to react permitting a pure Thermit reaction to take place. After the ethyl alcohol has evaporated and at the higher temperatures the Thermit reaction will be initiated within the pores of the body, the resulting heat generated will cause the free iron to fuse to the base metal clogging the pores and decreasing the body permeability;

Tests conducted upon specimens treated with the above method of impregnation have shown decreases in the permeability coeflici'ent from 345 cm. 10 before treatment to .14 cm. 1() after firing. The. minimum permeability occurs when the metal body is fired at 1100 F. for one hour. Firing temperatures lower and higher than 1100 F. for one hour do not provide optimum resistance to leakage as the Thermit reaction does not seem to be completed at lower temperatures and a breakdown occurs at higher heat. However, by firing the metal body at 1000 F. for 16 hours a permeability coefficient equal to that obtained by firing at 1100' F. for one hour is achieved.

It is thus understood that by theabove treatment utilizing readily available low cost ingredients a casting or other metallic body of inexpensive material may be treated to decrease the porosity thereby permitting the components of high pressure systems to be constructed of low cost material rather than expensive alloys.

We claim:

In a method of decreasing the porosity of metal surfaces,-the improvement comprising the steps of subjecting a metal surface to subatmospheric pressure, immersing said surface while under said subatmospheric pressure in a suspension of a finely powdered reaction mixture consisting essentially of iron oxide and aluminum in a volatile liquid carrier, subjecting said surface while so immersed to superatmospheric pressure, removing said surface from the subjection to said superatmosphericpressure, and heating the thus treated said surface to a temperature sufiicient to initiate reaction of said mixture.

References Cited in the file of this patent UNITED STATES PATENTS 

